Ammonium sulfate has a multiplicity of applications. For example, ammonium sulfate is used as fertilizer or fertilizer additive. In this case ammonium sulfate is a source not only of nitrogen, but also of sulfur, which are important plant nutrients. In many soils worldwide, there is a sulfur deficit which can be compensated for at least in part by targeted addition of ammonium sulfate.
Ammonium sulfate can be produced in various ways. For example, ammonium sulfate can be formed by introducing ammonia into sulfuric acid. Industrially, ammonium sulfate is frequently crystallized from solutions which arise as a by-product, for example in coal furnaces or plants for producing caprolactam. In the crystallization of ammonium sulfate, usually square] crystals form which customarily have a diameter from 1 to 2 mm.
Ammonium sulfate is customarily not the sole constituent of a fertilizer; rather, fertilizers comprise combinations of various plant nutrients (such as, e.g., nitrogen, phosphorus, potassium or sulfur). Ammonium sulfate, in use, is therefore frequently mixed with granulated fertilizers in order to produce a balanced fertilizer mixture.
However, crystalline ammonium sulfate has some disadvantages that make incorporation thereof into granulated fertilizer mixtures more difficult. Firstly, the ammonium sulfate particles formed in the crystallization are relatively small, secondly the particles frequently vary greatly in size thereof owing to abrasion and dust formation.
These properties make it difficult to produce physically homogeneous fertilizer mixtures with ammonium sulfate. In the distribution of fertilizer mixtures, however, uniform mixing and grain size distribution of the individual constituents is essential. An excessively broad grain size distribution can, in addition, also lead to mechanical problems in the uniform discharge of the fertilizer mixture.
For these reasons, more and more frequently, granulated fertilizers or fertilizer mixtures are used which, in addition, can be provided only shortly before use by mixing the individual constituents. Granulated ammonium sulfate is ideally spherical, and the individual granule particles have a diameter, for example, from 2 to 4 mm. This size is based on urea granules, which is the most widespread fertilizer worldwide.
For production of granulated ammonium sulfate, various processes are known in the prior art.
U.S. Pat. No. 4,589,904 describes the granulation of ammonium sulfate in a drum granulation with downstream dryer, wherein the solution is produced in a preneutralizer.
US 2012/0231277 relates to the production of built-up granules by fluidized-bed or spouted-bed granulation. For this purpose, granulation nuclei, which have been produced in advance separately, are sprayed with an ammonium sulfate-containing solution and then dried.
A problem in the granulation of ammonium sulfate is the formation of dust, whereby particles having a diameter of less than 0.5 mm are understood. The formation of dust is substantially due to three sources. First, the nozzles that atomize the material that is to be granulated generate in each case droplets having a certain distribution of diameters, wherein some of the finest droplets solidify before they impact the ammonium sulfate particles, in such a manner that the dust thus formed leaves the granulator again together with the exhaust air. In addition, the abrasion of the granules, owing to movements and collisions of the particles, may be mentioned as a dust source, in particular in a fluidized bed, wherein the amount of the dust produced depends significantly on the mechanical properties of the granules. Finally, a third source that may be mentioned is the dust formed from the comminution of excessively large granule particles, which dust, in the processes and plants of the prior art, is customarily transferred directly back into the granulator.
For this reason, granulating additives are frequently used that are intended to reduce this dust formation. The addition of these additives leads to the granule particles, and in particular the surface thereof, remaining plastic, in such a manner that as a consequence of their rolling motions and collisions, predominantly round particles having a smooth surface and good mechanical stability are obtained. The resultant granules therefore have a high compressive strength and impact strength, a low tendency for dust formation due to abrasion, and, furthermore, even in the case of relatively long storage, only a small tendency toward lump formation. Corresponding granulation additives, however, are used not only in fluidized-bed granulation, but also in other processes, such as, for example, spray crystallization or drum granulation.
A variety of processes have been described in the prior art for the avoidance or reduction of dust formation. Wang et al. (Particuology 11 (2013), 483-489) describe the use of calcium carbonate or silicon dioxide as additive in the ammonium sulfate solution that is to be granulated, wherein the respective additive must be used in a relatively large amount in order to obtain a satisfactory granulation result. Since these two additives are virtually insoluble in water and form a suspension, they may only be used as microparticles or nanoparticles, which, in turn, is a considerable cost factor in the procurement thereof. Secondly, excessively coarse particles could damage the pump and block the spray nozzles.
The processes and the devices for granulation of ammonium sulfate are, however, not satisfactory in all respects, and there is a need for improved processes and devices.